[RC4] Ruined Dungeon guardian killed by other mob?

[Torokasi]

Was exploring Ruined Dungeon, cleared everything I could find easily; got to the end and realized I'd only found five of the six guardians. The sixth room had only two Luminous Horrors (odd since they spawn in threes) and no rare.

Checking the log, six guardians did spawn; the sixth was a Wyrmic with Burrow, so I dug around the entire zone on the chance he got teleported into a wall somewhere. No such luck. My only guess is that the Horrors and the guardian fought, and the guardian killed one horror before dying? It's the only thing I can think of, here, since it's nowhere else on the floor.

EDIT: Log available of the Ruined Dungeon dive if it's wanted.